Out of those three variants of poker for me the best thing to play are MTTs. The prize pool in MTTs is much higher than the prize pool of sit and go`s and spin an go`s. Of course with proper bankroll management and optimal play one can benefit big time from playing MTTs. The advantage of playing MTTs is that if you win one MTT or get to the final table you will significantly boost your bankroll. It is hard to win an MTT but if you play many MTTs like at least ten a day, there is good chance that you will get a good monthly or yearly income from your cashes at MTTs if you play well and avoid tilt or playing on tilt. Now if your bankroll is short the best thing to do is play sit and go`s to build the bankroll needed to play MTTs. I would suggest having a bankroll of at least 100 buy ins for MTTs.
In that way I will compare the three types of the tournaments to play. It is best to play in SNG, especially if are newer to the game. Also SNG are not so long and are much easier to cash than MTTs. In MTTs, if you are playing slower tournament with a lot of minutes between the blinds, it is very hard and exhausted to get into the cash and if you can not get into the money, it is easily to start tilting.
Dont play SpinAndGo, if want to be a serious player.
Just find a game that you can beat, and play a lot.
The least variance has SitAndGo, maybe cash games too.
In MTTs you need a large bankroll and time frame etc.